This invention relates to a cureble composition of a halogen-containing polymer containing, as a crosslinking agent, a 2,4-dithiohydantoin compound or a metal salt thereof which can readily and effectively induce curing of the halogen-containing polymer. Cured articles from the composition have various superior properties such as a high curing density, good heat-aging resistance and good dynamic fatigue resistance.
More specifically, this invention pertains to a curable composition of a halogen-containing polymer composed of
(1) 100 parts by weight of a halogen-containing polymer,
(2) as a crosslinking agent, about 0.1 to about 10 parts by weight of a 2,4-dithiohydantoin having the formula ##STR2## wherein R.sub.1 and R.sub.2 are identical or different and represent a member selected from the class consisting of a hydrogen atom, C.sub.1 -C.sub.8 alkyl groups, C.sub.6 -C.sub.8 cycloalkyl groups, C.sub.2 -C.sub.8 alkenyl groups, C.sub.6 -C.sub.8 aryl groups and C.sub.7 -C.sub.8 aralkyl groups, or R.sub.1 and R.sub.2 are bonded together to form a 5- to 7-membered hydrocarbon ring which may contain a nitrogen atom and have at least one lower alkyl group as a substituent, or a metal salt thereof, and
(3) as an acid acceptor, about 0.5 to about 50 parts by weight of a compound of a metal of Group II or IVa of the periodic table.
Halogen-containing polymers such as polychloroprene, polyepichlorohydrin, an epichlorohydrin/ethylene oxide copolymer, chlorine-containing acrylic rubber, chlorinated polyethylene, brominated butyl rubber, fluorine rubber and polyvinyl chloride in the cured state find extensive use as materials having good properties, such as superior thermal stability, oil resistance and chemical resistance. It has been difficult however to cure the halogen-containing polymers effectively because the carbon-halogen bond in these polymers is chemically stable.
Numerous curing agents for halogen-containing polymers have been suggested heretofore, but none have shown a satisfactory curing effect. 2-Mercaptoimidazoline, a typical curing agent now in commercial use, too, has the defect that it cannot cure polymers having relatively low reactivity, such as chlorinated polyethylene or polyvinyl chloride, at a feasible speed. In addition, it has been pointed out that 2-mercaptoimidazoline may be carcinogenic (P. R. Johnson, Rubber Journal, pages 37-44, April 1973).
In view of the state of the art, it has been desired to develop other curing agents of industrial value which can easily and effectively cure halogen-containing polymers having relatively low reactivity.